Chimney liner

A chimney liner, in Slovak komínová vložka, is the new inner lining that carries combustion gases up through a chimney. Older brick and masonry chimneys often have cracked flues, open joints, or passages that are too wide and too cold for a modern appliance. A liner gives the gases a smooth, sealed route, keeps hot and corrosive gases away from the masonry, and stops smoke and carbon monoxide from leaking into living spaces. The liner forms the chimney flue, the passage itself, so relining replaces the lining without rebuilding the whole chimney body.

What does a chimney liner do in an existing brick chimney?

In a renovation, the liner is a practical way to get a safe flue that matches the new appliance. The old flue is measured and inspected first, because the liner must fit the appliance outlet and the actual route, including any bends. Relining does not change the outer walls or the chimney head, so it suits a chimney whose body is sound but whose inside no longer works.

Which liner materials are used in renovation?

Five liner types are common in Slovak renovation work. The right one depends on the appliance, the shape of the existing flue, and whether the liner can be installed from the top, from the bottom, or through a cleaning opening. The table gives the general picture only. The product must suit the appliance manufacturer's instructions and the designer's calculation.

Liner type	Typical use	What to check
Flexible stainless steel	Pulled down an existing flue, handles bends and irregular passages	Steel grade, insulated or uninsulated, sealing at every joint
Rigid stainless steel pipe	Straight, accessible flues, often insulated	Needs access for assembly and proper support at the top and bottom
Ceramic or clay liner	Traditional masonry flues, mainly for dry operation	Joint sealing, weight, and careful handling of fragile sections
Cast-in-place liner	Formed in place inside a masonry flue	Messy and hard to reverse, so the old flue must be sound first
Plastic (polypropylene) liner	Condensing gas appliances	Approval for the appliance type and its flue gas temperature

A liner that is oversized or undersized loses draft and lets flue gases cool too early, which causes condensation and soot even when the material is right. Sizing belongs to a designer or chimney specialist, not to a general rule of thumb.

Why does a condensing gas boiler need a different liner than a wood stove?

The difference comes down to wet or dry operation and to what the flue gas carries. A condensing gas boiler cools its flue gas so far that water vapour turns to liquid inside the chimney. That condensate is mildly acidic, so the liner must resist it, its joints must hold, and the chimney must drain the liquid at its base. A wood stove runs dry but very hot, and it leaves soot and creosote, a tarry deposit that builds up in a cool or slow-drawing flue. If that deposit ignites, the liner has to survive a chimney fire without cracking or letting the heat reach the structure.

Point	Condensing gas boiler	Wood stove
Flue gas condition	Wet: condensate forms inside the liner	Dry, but hot, with soot and tar
Main risk	Corrosion and condensate leaks	Creosote build-up and chimney fire
Liner needs	Condensate resistance, sealed joints, drain at the base	Heat and soot-fire resistance, usually insulated
Typical material	Stainless steel of a suitable grade, or an approved plastic system	Stainless steel built for solid fuel, or an approved ceramic liner

A liner approved for one appliance type is not automatically approved for another. Before choosing a product, check its stated duty against the boiler or stove manual and the designer's specification.

How is a chimney relined during a renovation?

Relining follows a fixed order, and skipping a step usually means redoing work. The sequence below is the usual practice in Slovakia, described in general terms.

1. Inspection: A chimney specialist (kominár) inspects the flue, often with a camera, and records cracks, open joints, deposits, and the existing draft.
2. Asbestos check: The old flue material is identified before anyone cuts, drills, or pulls anything. Asbestos-cement pipes are covered in their own section below.
3. Design: A designer selects the liner type and size for the appliance, the fuel, and the route through the building.
4. Installation: The liner is fitted, joints are sealed, and a condensate drain is placed where the appliance requires one.
5. Testing and approval: The flue is checked for draft and tightness and approved by the specialist before the appliance is connected.

What about asbestos-cement flue pipes from the socialist era?

Many houses built or extended during the socialist era have flues made of asbestos cement, known in Slovak as azbestocementové dymovody. Asbestos fibres are hazardous when the material is cut, drilled, broken, or sanded, because the fibres can be breathed in. Intact cement is less dangerous than damaged cement, but a pipe should never be assumed safe just because it looks intact.

Asbestos-cement pipes must be removed by a firm licensed to work with asbestos. They must not be cut, dismantled, or drilled on site by the owner or an unlicensed crew. The licensed firm removes the material, packs the waste, and delivers it to an authorised disposal site, as described for construction and demolition waste containing hazardous materials. A new liner does not make the old pipe safe on its own. Whether the old pipe is left in place, sleeved, or removed is a decision for the licensed firm after inspection.

What are the common mistakes with chimney liners?

• Relining hides a damaged body. A liner protects the passage and improves draft, but cracks in the outer masonry, damp, or a leaning chimney need their own repair first.
• Any stainless steel will do. Steel grade, insulation, and design must match the appliance. A liner suited to a wood stove is not automatically suited to a condensing boiler.
• A flue that draws well is sound. Good draft can coexist with leaks or with cold walls that condense flue gas.
• The asbestos pipe can be trimmed to fit. It cannot be trimmed safely by an unlicensed person, and the job belongs to a licensed firm.